BACKGROUND TO SPACE WEATHER - A TUTORIAL

by John A Kennewell

The material here is a modified form of an invited
student tutorial presented by the author at the Conference on
Solar and Terrestrial Physics (STEP 1995) held at the University
of Adelaide from 25 to 29 September 1995.

1 INTRODUCTION

The concept of space weather, although not a new one (see Kennewell,
1984), has recently risen to prominence as an overview term for the
processes that occur in and give rise to the space environment. It
encompasses all of solar-terrestrial physics, and particularly
includes the applications arena wherein our hi-tech and space
systems both use and are affected by the space environment.

The current emphasis on the term 'space weather' originated in
the USA, culminating in a National Space Weather Program. The
aim of this drive is to raise the profile of space environmental
studies and services, to educate, to unite more closely a diverse
community, and to produce the visibility and understanding necessary
for successful funding of a suite of viable programs.

SOME DEFINITIONS OF SPACE WEATHER

From the US National Space Weather Program

"Space Weather" refers to conditions on the Sun and in the solar
wind, magnetosphere, ionosphere, and thermosphere that can influence
the performance and reliability of space-borne and ground based
technological systems and endanger human life. Adverse conditions
in the space environment can cause disruption of satellite operations,
communications, navigation, and electric power distribution grids,
leading to a panoply of socio-economic losses.

A Physicist's Viewpoint

Space weather is the application of space environmental studies to the
interactions that occur between the space environment and the
activities of humankind.

A Personal Viewpoint

Space weather is the state of the space environment, the appreciation
of that environment and the effects that environment has on our
activities and ourselves.

2 THE SCOPE OF SPACE WEATHER

2.1 Solar Driven Space Weather

The primary driver for most of our space weather is the Sun. Processes
within this star give rise to phenomenon that travel outward through
the interplanetary medium and which, if travelling in the right direction,
may couple into the Earth's magnetosphere and ionosphere.

On a calm day the solar output ranges across the electromagnetic spectrum.
The emission peaks around 500nm in the visible spectrum, and extends
at decreasing intensity into the lowest reaches of the radio spectrum
and into the X-ray spectrum. On Earth, the heat and light make all biological
life (the ecosphere) possible. The UV and X-ray emission are
absorbed at various heights in the atmosphere, giving rise to the
ozone layer and the ionosphere. The latter is used for communications,
navigation and remote sensing.

Particulate radiation is also emitted by the Sun, principally due to
the million Kelvin solar corona (outer atmosphere). These particles,
typically electrons and hydrogen and helium nuclei, travel out through
the interplanetary medium as the solar wind, with an average speed
of 400 km/sec. At this speed the wind is hypersonic with respect to
the Earth and a bow shock wave is set up. The Earth's magnetic field
is altered through this interaction forming the well known magnetospheric
shape. This magnetosphere acts as a partial shield to incident
particulate radiation, but allows greater access to Earth at the polar
cusp, and via the magnetotail for low energy particles.

In quiet conditions solar particle energies are generally less than
about 1 MeV. At Earth they produce radiation belts with ring
currents girding the planet. On the ground the diurnal variation can
be measured in the geomagnetic field.

Both the e.m. and particulate radiation from the Sun vary in space
and time. Temporal variations have periods of milliseconds to hundreds
of years. One prominent variation, involving the manifestation of
dark spots on the solar disc, has a period of about 11 years.
For much of space weather, this period is a dominant one.

We currently believe that magnetic fields within the Sun are the
controlling elements of solar weather. These give rise to the
phenomena of sunspots, plage, filaments and coronal holes. They
are also responsible for the solar activities discussed below.
On a stormy day, the Sun's output in the radio and X-ray spectrum can
increase by several orders of magnitude. Radio energy can cause interference
to sensitive systems, particularly wide beamwidth, low link margin
satellite L-band mobile communication systems. The X-ray increase
causes a 'thickening' of the lower ionosphere that effectively absorbs
most of the high frequency (HF) radio energy incident upon it. This
may effectively destroy all short wave communications, interfere with
some navigation systems and blanket over the horizon (OTH) radar
systems. Increases in the total electron content of the higher
ionospheric layers can reduce the precision of the GPS navigation
system, and even in certain parts of the world, render satellite
communication unusable due to ionospheric scintillation. Space
tracking systems experience ranging and positioning errors.

Explosive events that occur on the Sun are powered by
reconfiguration of solar magnetic fields (eg magnetic reconnection).
These include flares, coronal mass ejections and solar particle events.

Solar flares
are abrupt increases in electromagnetic radiation,
most notably in the radio and X-ray regions of the spectrum.
Total energy release in large solar flares may exceed 1027
Joule (equivalent to 200,000 million megatons of TNT). The X-ray
energy causes increased ionisation and warming at the base of the
thermosphere (~100km), and the radio energy can interfere with a
multitude of Earth and space-based sensors and systems.

Solar flares are seen from the ground in the light of
the hydrogen alpha spectral line. Credit: BBSO

Coronal mass ejections
(CMEs) are the acceleration and release of
giant clouds of plasma containing low energy particles. The plasma
in these clouds constrains and carries some of the solar magnetic
fields. The fields are effectively frozen into the plasma. The mass
of a large CME can exceed five billion tons, and the energy associated
with the largest CMEs is comparable to the energy associated with
the largest flares.

Arriving at the Earth's orbit these plasma clouds compress the Earth's
magnetosphere. At times the magnetosphere may be compressed below
geosynchronous orbit, exposing satellites there to the direct solar
wind. If the interplanetary magnetic field is directed south, large
quantities of low energy particles (electrons and protons) may be
injected into the magnetosphere, especially at the magnetotail.
This results in the expansion of the auroral oval equatorward. High
fluxes of electrons can cause severe satellite differential charging
and deep dielectric charging. Subsequent discharges can destroy
satellite systems.

Magnetic and ionospheric storms can disrupt communications, navigation
and other systems over a wide area for many days. If the rate of
change of the magnetic field is great enough, currents may be induced
in long cables and pipelines, causing disruption to power services, damage to
equipment, and corrosion in metallic pipes. Some believe that
animal navigation and migration dependent on the geomagnetic field
may also be affected.

Solar particle events
(SPEs/SEPs) release large numbers of high
energy (relativistic) particles, predominantly protons with
energies up to 1 GeV. These particles may arrive at the Earth
within a few tens of minutes and can degrade solar cells, cause
computer memory upsets, and pose a severe radiation danger to men
and machines in vast areas of space. The highest energy SPE events
can be detected at ground level (GLEs) and can cause significant
radiation increases at aircraft cruise altitudes. They may well
prove to be a limiting factor in manned exploration of the planets.

The relationship between the three solar energetic phenomena is
still a matter of lively debate. The largest magnetic reconfiguration
events tend to produce all three, but smaller energy releases may
result in only a flare or only a CME.

SPEs are the most dangerous and damaging of all solar energetic
activity, and are fortunately the most rare.

2.2 Extrasolar Space Weather

The Sun is the major, but not the only source of space weather.
Galactic cosmic radiation has been known and studied for many decades.

Meteor burst communication is currently used in a number of applications
where traditional ionospheric or satellite communications are not
appropriate. On the debit side, a large influx of meteoric material
can seriously degrade the operation of an OTH radar system.

Small meteoriods (the interplanetary body that produces the meteor
phenomenon) pose a small hazard to satellites and spacecraft
(particularly large long lived operations), although man-made orbital
space debris is now a more significant hazard.

Much larger individual meteoroids (>100m) can pose a hazard to the
ecosystem, and bodies larger than 1 km hold the import of global
catastrophe (Steel, 1995).

There is evidence that extrasolar X-ray and gamma ray sources may
produce a measurable increase in the lower ionosphere, an increase
detectable in VLF signal propagation. There is also growing
satellite sensor evidence that our magnetosphere contains nuclei
and ions initially injected into the heliosphere as it ploughs through
the periphery of a giant interstellar cloud.

A supernova occurring anywhere within a thousand parsecs of the solar
system could create very stormy weather indeed, and would make life on
the Earth quite uncomfortable.

A recent speculation on the nature of the enigmatic gamma-ray bursts
ascribed them to the merging of neutron stars in binary systems. If
such a collision were to happen in our galaxy, the intense gamma-ray influx
to the lower mesosphere and stratosphere may produce enough nitric
oxide to totally destroy the ozone layer, and enough nitric acid to
produce years of high level acid rain.

There is indeed more to space weather than what is under the Sun!

3 AN INTEGRATED OVERVIEW

Provision of a Space Weather Service is essentially one of being able to
specify, both for the present and into the future, the energy, mass
and momentum fields throughout the near space environment. That
environment is currently of interest for the Sun to the Earth, but
even today, occasionally requires specification past the Earth, to Mars
and even to the outer solar system.

We observe, and our observations are a basic input for our research.
Guided by physical laws and a measure of our own speculation, the
research leads to understanding. Understanding gives us models to
attempt prediction, and it also guides the development of instrumentation
to extend our observational capabilities.

The observations simultaneously allow a real-time Space Weather Patrol.
This allows nowcasting, and together with the models, forecasting
of the space environment for a diverse array of customers whose
systems operate by, through, and in that environment.

An essential part of this process, if it is to be successful, is
feedback between each group. Forecasters need the researchers, but researchers
must monitor the forecast output to validate and refine models.
Customer - forecaster - researcher interaction is required for each
to be able to understand specific system-environment interactions.
The customer must be able to interpret the forecast, understand the
consequences, and maximise system function. The forecaster and
researcher need to tailor model outputs for the specific situation.
This whole area is one that deserves more attention. It is an
essential directive for a successful Space Weather Service. It need
not be restrictive upon researchers. New areas of exploration should
ideally precede and anticipate customer demand, but in reality,
system interactions can also lead to unexpected phenomena demanding
investigation.

Figure 1: An Integrated Space Weather Service

4 BASIC SPACE PHYSICS

There are, of course, a wide range of physical laws that govern the
behaviour of the matter and energy that pass through, interact in,
and constitute the space environment. Each specialist undoubtedly has
his or her favourite equations, and en masse these present a formidable
barrier to the novice space meteorologist. There are however, a few
simple relationships which help us to distinguish and understand
general types of space environmental behaviour.

4.1 The PB Ratio

The space environment is permeated by plasma and magnetic fields. The
behaviour of this combination can be dichotomised by a ratio (which
we shall term PB) of the plasma kinetic energy density to the magnetic
field energy density:

PB = ( ½ ρ v2 ) / ( B2 / 2μo )

where ρ is the plasma mass density, v is the ion rms speed, and B
the magnitude of the magnetic field. Calculation of this ratio gives
a knowledge of the type of behaviour that can be expected in the
given conditions. When PB>1 the charged particles have dominant control,
and when PB<1 the magnetic field control is dominant. In the solar
photosphere PB=106, in sunspots 0.5, in the solar wind 4,
in the magnetopause 1 (virtually by definition), and in the far
magnetosphere 0.1 .
[As an instructive exercise evaluate PB for the various ionospheric
layers]

4.2 Plasma Frequency

This relationship crops up all over the place. It is equally pertinent
in the solar corona as in the Earth's ionosphere. It allows the
interpretation of a multitude of phenomena and governs the allowable
operation of several systems. The fundamental plasma frequency
(also known as the plasma critical frequency) is given by:

fp = ( e / 2π ) √ ( Ne / me ε )

where e is the electronic charge, Ne the electron number
density, and ε the plasma permittivity. This equation defines
the natural resonance frequency of a plasma. Below fp the
plasma is essentially opaque to e.m. radiation; above it is often
stated to be transparent, but it might be more accurate to say
translucent, particularly just above fp.
[Exercise - derive this frequency from fundamental laws]

4.3 Particle Motions

The magnetosphere is one of the most important areas of space
environment research and space weather effects.

The Earth's magnetic field acts like a giant cocoon, to shield us
from much of the ambient space radiation. However, not only very high
energy particles can penetrate this defence, but vast clouds of low energy
material can break through the back door of this shield.

The fundamental equation describing the motion of charged particles in
a magnetic field is the Lorentz force equation, which when combined
with Newton's second law of motion can be written:

m dv/dt = q ( E + v x B )

One of the simplest, but practically useful solution of this equation is
cyclotron motion with a radius:

r = m v / q B

and a period of:

T = 2 π m / q B

Some other real solutions require large amounts of super computer time to describe.

5 GRAPHICAL VISUALISATION

The presentation of sensor data and the visualisation of model output
is an extremely important component of space weather. Even more than with
terrestrial weather, we are dealing with a 'deep' three dimensional
environment. Numerous time varying fields exist throughout this
environment. Sensors are scarce, and data points must often be used
as very limited boundary inputs to a model that is expected to
extrapolate behaviour over an incredibly large volume.

Visualisation of the Earth's magnetopause location
Credit: IPS Radio and Space Services

It is very easy to develop tunnel vision in a field such as this. Extensive
graphical visualisation is often the key to assist researchers and
particularly forecasters come to grips with the nature of the spatial
and temporal variations. In real-time customer support, it is
absolutely vital to be able to sort out the 'signal' from the noise.
What is noise to one system may be all important to another. Dynamic
integrated displays giving a situational overview with rapid recognition when
parameters exceed preset event thresholds are most desirable.

Visualisation is not only important, but is usually responsible for
vastly more lines of computer code and computer time than the basic model
whose output is displayed. In the mid 1990's the US Air Force awarded
a multimillion dollar contract to a commercial firm purely for
graphical visualisation of model at a space forecast centre in Colorado.
In at least one of the programs, the actual model could be described by
about a dozen lines of code embedded within many thousands of lines of
the display routines.

Fundamental to the graphical visualisation dilemma is the need for new
ideas of presentation. Computer code can only implement what first
originates in the human mind.
[Exercise - Display the auroral oval. Reference Jursa (1985) chapter
12 for positional algorithms and data. Use the magnetic K-index as
the only space weather input.]

One visualisation of the auroral oval
Credit: IPS Radio and Space Services

6 SPACE WEATHER SENSORS

In terrestrial meteorology, there are two parameters that appear to have
fundamental importance: temperature and pressure. In space meteorology
there are similarly two indices that are widely used as weather
indicators: solar 10cm radio flux (F10) and the geomagnetic
K-index (Kp). Sometimes sunspot number (SSN) which is
highly correlated with F10, and the Ap index, which is
derived for Kp are also used.

These two indices are ubiquitous in the field. They are used as inputs
to ionospheric models, auroral behaviour, satellite lifetime and orbital
decay predictions and many other space weather predictors.

SSN and F10 are derived from measurements of the Sun at radio
and optical solar observatories, and a network of such observatories
now routinely makes these measurements. The K or A indices are derived from
ground-based geomagnetic observatories. Again, a network of stations
across the globe, now mostly automated, produces these values.

As there is a lot more to terrestrial weather than temperature and
pressure, so does space weather require a diverse array of measurements,
Solar imagery is desired at many different wavelengths. Satellite solar
X-ray images are particularly useful, and this data is just becoming
available. Watching CMEs as they travel outward from the Sun is an
extremely desirable goal, but is only routinely achieved by space-borne
white light coronagraphs for a few tens of solar radii out from the
Sun. New radio telescopes soon to come on line may allow CME tracking
further away from the Sun. These will use amplitude scintillations
of quasar signals to map the interplanetary plasma. Available solar
radio and X-ray data can also provide useful surrogates. Satellites
at the Sun-Earth L2 Lagrange point (1.5 million km from Earth in the
direction of the Sun) can provide almost an hour's warning of a
potential CME impact with the Earth.

Probably the most complex arena of our immediate concern is the
magnetosphere. Only a vast complex of ground-based and satellite-borne
instruments will help us unravel and monitor this interface between
the Sun and our near-space weather. New satellites coming on line
promise to give us limited magnetospheric imagery in the near future.

Ground-based ionospheric monitors are essential for a space weather service,
and it is important to realise that even when we think we completely
understand a particular region scientifically (not that we do at
present), we still need a monitoring service. Predictions of ground
weather are not possble without a multitude of observations from a
well-spaced observing grid. Neither should we expect valid forecasts
in space weather after decommissioning our observing networks. A good
example of this lies in meteor radar sensors. For a long time there
was no continuous meteor radar patrol conducted anywhere. Meteoroid
influx, however, is not constant. The Earth may intercept a new stream
at any time, and confound certain systems. Fortunately a new class
of meteor radars are now being deployed. Routine patrol monitoring of
all space weather characteristics is important.

Until recently, predictions of solar activity have been made only
through observations of the 'surface' layers of the Sun. It is now
becoming possible, through sensitive helioseismology sensors, to examine
the solar interior. Instruments now on-line or being deployed on
the Earth and in space will hopefully open a new vista to the main
driving force of space weather.

7 OPPORTUNITIES

So you want to be a space weather person? Well, the field is
certainly diverse, not only in the subject matter, but also in the
application. Many Australian universities have basic and/or
applied research areas that encompass the disciplines important to
space weather, from plasmas through astronomy and astrophysics to
communications, navigation, remote-sensing and space operations.

Probably the first step is to decide on your area of interest, read
all you can, and then attempt to become attached to a group that is
going where you want to go.

IPS Radio and Space Services is the Australian Agency (www.ips.gov.au)
that provides space weather monitoring, coordination and advice to a
wide range of customers in the Australasian region. IPS maintains
connections with space weather research and service bodies throughout
the world. IPS also frequently advertises positions. The minimum
requirement is usually a B.Sc. with a demonstrated interest in space weather.
Watch the news box in the IPS home page.

As discussed, there are many fields within space weather. At the
present time, high profile areas are coronal mass ejections,
magnetospheric physics and helioseismology applications.

If you are interested in satellite and space operations, you will
need to travel overseas, and you should plan for and pursue this as early
in your career as possible. Funding for space weather comes
primarily from governments. In the USA, a high degree of funding
comes from the military, and a small component also comes from
customers such as power companies. Universities and space agencies
in Europe, Japan and the USA are the most likely places to persue
space weather research.

If you are looking for ideas or modelling exercises, the Space
Weather Journal published by the American Geophysical Union (AGU)
is a good place to start (the quarterly printed copy is free upon
application, but the full e-journal access is required to read all
the scientific articles). Although no longer held, the five
yearly Solar Terrestrial Prediction Workshops published large volumes
of proceedings from 1979 to 1995, and these are very much worth
reading for practical ideas.
Space weather data can be obtained from IPS and the hosted World
Data Centre for Solar Terrestrial Science. Links to other data
centres around the world are also given.

8 MODELLING/VISUALISATION EXERCISES

Presented below are three student exercises in the area of space
weather modelling and visualisation. They range from simple to more
complex, although there is undoubtedly a vast visualisation space
in which they might be presented. Actual data to test the models
may be obtained from the World Data Centre for Solar Terrestrial
Science at IPS Radio and Space Services (www.ips.gov.au) and the
US Space Weather Prediction Center and the US National Geophysical
Data Center.

8.1 Solar X-ray Ionospheric Effects

Solar x-ray flares produce an increase in the ionisation of the D-layer
(60-90 km altitude). This causes absorption of HF radio signals
proportional to the inverse square of the frequency of operation. At
any point on the Earth, the lowest useable frequency at a given
geographical position in the ionosphere is roughly specified by the
formula:

fL (MHz) = 174 ( X + 1.86x10-6 )1/4 cos3/4 χ

where X is the solar x-ray output in watts per square metre
and χ is the solar zenith angle.

The graph below also plots the highest frequencies anywhere likely
to be affected and the expected duration of the effect, given the
peak x-ray flare intensity. (Note that this is an entirely empirical
model).

Visualise this effect, both in space and time, for a customer who
might have multiple HF communication systems operating throughout
the world.

8.2 Interplanetary Shock Time of Arrival

Following a coronal mass ejection, the plasma cloud travels through
the interplanetary medium. Indication of this may be given by various
radio spectral signatures. The model descibed by Smart et al (1986),
computes the time of arrival of the shock wave preceding the CME at
the Earth's orbit, and provides an indication of the magnitude of the
shock.

Visualise the shock throughout space with emphasis on terrestrial
interception. Use windows around the space visulation to indicate
prediction at the Earth and/or specified comet/spacecraft locations.

8.3 Proton Prediction

In large solar magnetic reconfiguration events, relativistic protons
are produced (and are the most dangerous of all space weather events).
Solar radio data signatures and intensities have been used as
indicators of proton production. Shea and Smart (1979) have
desribed a model that allows the prediction of proton flux as a
function of time and proton energy at geosynchronous orbit. The paper
should be referred to for the necessary background and equations.
Although a little dated and of questionable accuracy, it is still the
only such model available. An additional paper by Bakshi and Barron
(1979) will allow the proton energy exponent to be computed.

Sensor input is observation of a type II/IV radio spectral complex
(as a binary decision on proton production), the peak 8800 MHz solar
radio flux during the event, and the Castelli-U omega ratio of the
microwave emission. All this data is available from radio telescopes
at the Learmonth Solar Observatory in Western Australia.

This is the most complex of all the three exercises, both with regard
to the model complexity and the visualisation component. It could
range from a simple time-energy graph at geosynchronous orbit, to
a full 3-D space propagation display. Accuracy can be checked against
geosat proton flux graphs available in real-time at the US Space
Weather Prediction Service.